Cholesteric liquid crystals (CLC) may be prepared from mixtures of cholesterol directives or nematic liquid crystal material, which is combined with one or more chiral dopants. The natural self-assembled helical structure of CLCs enables the cholesteric liquid crystal molecules to twist into a helical structure. For example, cholesteric liquid crystals that are in a ground state have a twisted director field along a helical axis with a periodicity that is characterized by a cholesteric pitch (p) of the CLC at a rotation of the director in 360°. The helical pitch (p) may be tailored to circularly reflect an electromagnetic wave at a preselected wavelength with same optical handedness of the cholesteric liquid crystal. In addition, the cholesteric liquid crystal circularly transmits the other half of the incident electromagnetic wave with the opposite optical handedness.
Cholesteric liquid crystals (CLC) are particularly suitable for use in light modulation devices, or spatial light modulation (SLM) devices, because of their unique polarizer-free optical behavior, which includes optical bistability, color or Bragg reflection, and light-scattering modes/states that are controlled by the electric field-induced change in the liquid crystals. These optical modes/states may be switched back and forth (i.e. reversed) by the application of different external electric fields or different frequencies supplied by a voltage source. In order to observe the field-induced optical effects in cholesteric liquid crystals, the cholesteric liquid crystals are typically sandwiched between two parallel substrates having transparent electrodes deposited on the inside surface of each of the substrates. This configuration allows the electric field or voltage source to be applied across the top-down transparent electrodes. Alternatively, electro-optical cells for in-plane switching of cholesteric liquid crystals (CLC) may be prepared with an interdigitated electrode pattern that is disposed on one substrate and no electrode on the other substrate.
The unique field-induced optical effects produced by the CLCs include, for example, a change in optical states at a switched texture, a change in the switched helical pitch (p), or a change in optical spectra wavelength in response to an applied voltage. These effects depend on both the material properties of the CLCs and the configuration of the spectra light modulating (SLM) device utilizing the CLCs, including the surface treatment used by the SLM, thickness-to-pitch ratio (d/p), the dielectric anisotropy of the nematic host and the particular additives (nanoparticle, quantum dot, dichroic dye, polymer, etc.) used. Cholesteric liquid crystal based spatial light modulating device, which are based on polymer-stabilized cholesteric liquid crystals or polymer-dispersed cholesteric liquid crystals are especially suitable for commercial applications. For example, depending on the surface treatment or boundary conditions of the SLM, the cholesteric liquid crystals used therein may be prepared to have a planar alignment or to have no alignment, so as to reflect a preselected spectral wavelength, as well as be switched to a transparent state, a light-scattering state, or to a state to reflect another wavelength of light.
Another type of spatial light modulating (SLM) device may also be prepared using cholesteric liquid crystals with spherulite textures, whose light modulating effects are controlled based on the size of the gap of the cell used by the SLM device, as well as based on the helical pitch (p) of the CLC and its alignment. With a homeotropic alignment surface treatment and a cell gap (d) that is close or equal to the helical pitch (p), the treated alignment surface of the spatial light modulating device provides weak surface anchoring for the cholesteric liquid crystals, such that the helix deforms to form the spherulite texture. Thus, depending on the frequency of the voltage applied to the SLM device, the SLM device is able to take on a transparent state (in the case of the application of a high-frequency voltage) or an opaque state (in the case of the application of a low-frequency voltage). After the voltage is removed from the SLM device, the SLM device remains in a voltage-induced transparent state or opaque state, whereby the switched optical states are metastable at zero voltage.
In view of the forgoing, there is a need for a bistable liquid crystal spatial light-modulating device that does not require an applied voltage to obtain or maintain one of the following optical states: a transparent state, a light-scattering state or a light-absorbing state. There is also a need for a bistable cholesteric liquid crystal spatial light-modulating device that is based on a homeotropic (HO) and bubble domain (BD) texture change of the cholesteric liquid crystals, which is responsive to external stimuli, such as electric voltage, light irradiation and mechanical pressure or force. In addition, there is a need for a bistable liquid crystal spatial light-modulating (SLM) device that can be utilized in a variety of applications, including, but not limited to, sensors, smart windows, spatial light modulators and displays.